P0685 — ECM/PCM Power Relay Control Circuit/Open

The diagnostic trouble code P0685 indicates an issue with the Engine Control Module (ECM) or Powertrain Control Module (PCM) circuit related to the power relay control. Essentially, this code signals that the vehicle’s computer is detecting an open or malfunctioning circuit in the relay responsible for powering critical engine management components. While the specific labels may vary depending on the vehicle manufacturer, this trouble code generally points to an electrical problem that can influence engine operation and overall vehicle reliability.

System affected: ECM/PCM power relay control circuit
Severity: Moderate to high — can affect vehicle function
Main symptoms: Engine no-start, stalling, or electrical malfunctions
Driveability: Limited — may prevent vehicle from starting or cause erratic operation

Manufacturer variations for trouble code P0685

The P0685 code label can differ among vehicle manufacturers and their diagnostic systems. Often, manufacturers assign unique troubleshooting labels or codes that correspond to similar circuit issues. Here are some common variations:

Ford: P0685 — Control Circuit for Fuel Pump Relay
Volkswagen/Audi: P0685 — Starter Relay Control Circuit
Honda: P0685 — ECM Power Relay Circuit Malfunction
Toyota: P0685 — ECM Relay Control Circuit Error
BMW: P0685 — Relay Control Circuit for ECU

Always refer to the specific vehicle’s service manual or OEM diagnostic guide to confirm the precise label or description, as labelling can vary.

What does trouble code P0685 mean?

In simple terms, P0685 indicates that the vehicle’s ECM or PCM — the “brain” that controls engine and transmission functions — is not receiving the proper voltage or control signals from the relay responsible for powering it. The ECM relies on relays to safely manage high-current circuits; if a relay’s control circuit is open or malfunctioning, the ECM cannot power on correctly, leading to this fault code.

The code is triggered when the vehicle’s onboard diagnostic system detects an abnormality in the relay circuit, typically through a lack of expected voltage or continuity. This can happen during engine start-up, operation, or even intermittently, depending on the underlying cause. Such issues are often noted when the vehicle fails to start, stalls, or displays electrical warning lights.

Severity and risks of trouble code P0685

The P0685 code can vary in its impact depending on the vehicle’s design and the severity of the circuit issue. In some cases, it may simply cause the engine to fail to start or result in electrical malfunctions, such as issues with fuel delivery, ignition, or accessory operation.

However, if ignored, it could lead to more serious problems, including damage to relays or the ECM itself, or even safety concerns if critical control systems become unresponsive. Typically, this problem is considered to be of moderate to high severity.

**Can the vehicle be driven?** — Limited. While some vehicles may operate under certain conditions, it’s generally advisable to address relay circuit faults promptly to avoid potential breakdowns or further damage.

Symptoms of trouble code P0685

The presence of P0685 often correlates with several noticeable symptoms, though they may vary based on the degree of circuit malfunction. Many drivers experience one or more of the following:

Engine does not start or is difficult to start due to lack of proper power to the ECM
Engine stalls or cuts out while driving, indicating unreliable power supply
Electrical system warning lights, such as the battery or check engine light, illuminate on the dashboard
Loss of power to critical systems like fuel pump, ignition, or sensors
Intermittent operation or fluctuating engine performance

Most likely causes of trouble code P0685

This fault code typically stems from issues related to the relay circuit or associated wiring. While the actual root cause can vary, the most common causes include:

Faulty ECM/PCM relay: The relay itself may have become defective, corroded, or worn out, leading to an open circuit.
Wiring issues: Damaged, shorted, or corroded wiring, connectors, or terminals in the relay control circuit can interrupt proper operation.
Loose or poor electrical connections: Corrosion, dirt, or improper connection of relay sockets or ECM terminals can cause circuit opens.
Blown fuse: A fuse protecting the relay circuit may have blown, cutting off power supply.
Faulty ECM or PCM: An internal fault within the vehicle’s engine control module, though less common, can also trigger this code.

How to diagnose trouble code P0685

Diagnosis of P0685 involves systematic checks to confirm the circuit’s integrity and pinpoint the fault. Here are the generic diagnostic steps to follow:

Visual inspection: Check the relay, wiring, connectors, and fuses associated with the control circuit for signs of damage, corrosion, or loose connections.
Check fuse status: Verify that the relevant fuse is intact. Replace if blown.
Relay testing: Remove the relay and test it on an appropriate tester or substitute with a known good relay.
Inspect wiring continuity: Use a multimeter to test for continuity along the wiring from the relay control circuit to the ECM, ensuring there are no opens or shorts.
Voltage measurement: With the ignition on, measure voltage at the relay control circuit connector to confirm proper signals are present.
Monitor live data: Use a professional scan tool to monitor relay control signals, voltage levels, and circuit status in real-time.
Check the ECM/PCM: If all wiring and relay components are functional, further testing of the control module may be necessary, possibly involving OEM procedures or specialist equipment.

Possible repairs for trouble code P0685

Addressing P0685 involves correcting the underlying cause to restore proper relay control circuit operation. Repairs can vary from simple to complex, with some requiring professional assistance:

Replace the faulty relay: If the relay itself is defective or damaged, swapping it out for a new OEM part is often sufficient. (Basic repair — DIY if comfortable with electrical components)
Repair wiring or connectors: Fixing or replacing damaged wiring, cleaning corroded terminals, or securing loose connectors restores circuit continuity. (Basic to moderate repair — professional recommended if wiring is complex)
Replace blown fuse: Identifying and replacing the blown fuse helps restore circuit power. Ensure the root cause caused the fuse to blow.
Address ECM/PCM issues: If the control module is internally faulty, reprogramming or replacement might be necessary. This typically requires professional diagnosis and repair.

Vehicles commonly associated with trouble code P0685 in Europe

The P0685 code can be found in various vehicle brands, but its occurrence depends on the design and electrical architecture. In European vehicles, this issue tends to be more prevalent among models with complex relay control systems. Notable brands include:

Volkswagen Group: VW, Audi, Skoda, Seat — particularly models with advanced engine management systems.

Ford: Various models across their range, especially those with specific relay control modules.

Peugeot and Citroën: Certain petrol and diesel models that rely on relay-controlled circuits for engine management.

Frequent mistakes with trouble code P0685

Diagnosing and repairing P0685 can sometimes lead to overlooking key issues if certain common mistakes are made. Here are some pitfalls to avoid:

Ignoring simple fuse or relay problems: Fuses or relays are the most straightforward causes; skipping these checks prolongs diagnosis.
Replacing ECM/PCM prematurely: Internal control module faults are less common; replacing it without proper testing can be costly and unnecessary.
Overlooking wiring corrosion or damage: Electrical circuit issues often stem from wiring problems that are easily missed.
Not verifying the fault is consistent: Sometimes, the fault appears intermittently; driving patterns or environmental factors could influence diagnosis.

FAQ — trouble code P0685

Can this code disappear on its own?
Typically, no. If the underlying circuit fault persists, the P0685 code will remain until the issue is corrected. However, temporary glitches in the relay or wiring can sometimes cause transient faults that self-clear once resolved.

Can I keep driving?
While a vehicle may still be drivable temporarily, it’s strongly recommended to address the fault promptly. Continuing to drive with this code could lead to further electrical issues, engine stalling, or starting problems, especially if the relay circuit controls essential systems.

Why does the code return after clearing?
If the underlying electrical problem — such as a faulty relay, broken wiring, or internal ECM fault — remains unaddressed, the code will reappear after clearing. Persistent issues require thorough diagnosis and repair to ensure long-term resolution.